1. Field of the Invention
The present invention relates to a wireless modem, and more particularly, the present invention relates to a wireless modem that improves link characteristics between modems during periods of reduced environmental degradation of the link, and method therefor.
2. Description of the Related Art
Although there are several link dependencies that must necessarily be taken into consideration during the transmission of a carrier signal from a wireless transmitter to a wireless receiver, channel capacity is primarily dependent upon signal-to-noise ratio, or "SNR". Typically, as the SNR decreases, the channel capacity decreases, causing a link formed between the transmitter and the receiver to be degraded, corrupting the transfer rate of the corresponding carrier signal. On the other hand, as the SNR increases, the channel capacity increases, resulting in improved transfer rate of the carrier signal.
At the same time, while there are several factors that have a tendency to cause the SNR to decrease, environmental degradation, such as rain, snow, fog, and other non-transient man-made interference sources tend to be major factors causing a decrease in SNR. For example, individual raindrops absorb/scatter energy from radio waves and a certain amount of energy in the waves is scattered away from the propagation path. Rain attenuation and depolarization of a transmitted carrier signal particularly occurs during periods of intense rainfall, causing the SNR to degrade.
The level of the effects of these interactions between the carrier signal and the rainfall depend on both the number of raindrops encountered by the carrier signal, and the distribution of the size and shapes of the raindrops, both of which depend on the rate of the rainfall. In a wireless modem operating over a carrier at millimeter wave frequencies, where the wavelength of the carrier is close to the size of a raindrop, or on the order of a couple of millimeters, a raindrop is substantial enough in size to degrade the link during periods of moderate to intense rainfall. When the wireless broadband link is a terrestrial link, the entire link may be covered in rain, depending on the size of the associated storm, and therefore substantially the entire link is degraded.
As a result, in order to insure successful data transmission along a link when implementing wireless broadband links in the wireless modem, it is important that the links be engineered to operate during the period of the year in which the rainfall is the most intense. Therefore, since the most intense rainfall occurs typically during less than one percent of a given year, additional capacity of the carrier is available for more than ninety-nine percent of the time and cannot be used.
FIG. 1 is a graphical representation of a relationship between the SNR and rainfall over time. Environmental degradation of a signal that occurs, for example, during an intense snowfall in January is indicated by a downward extending spike 20a. In addition, environmental degradation of the signal that occurs during intense rainfall in June and July is indicated by downward extending spikes 20b-d, and environmental degradation of the signal that occurs during an intense snow storm in December is indicated by a downward extending spike 20e. Although degradation of a carrier signal due to intense snow or rainfall might only occur less than one percent of the time in a given year, for a link to be reliable it must be engineered to always operate throughout the year at an SNR corresponding to the periods of intense snow or rainfall. Accordingly, the link must be engineered to always operate at the lowest SNR, indicated by a horizontal line 22.
During the remaining ninety-nine percent of the year, when environmental degradation is no longer a factor, and therefore when the SNR that can be tolerated is greatest, indicated by a line 24, additional or excess capacity is available that cannot be used. This excess capacity is illustrated by a hashed region located between where the SNR can be tolerated, line 24, and the engineered level of the SNR, line 20. Therefore, the excess capacity is wasted ninety-nine percent of the time during the year, resulting in reduced throughput.